Beam-forming is a well-known technique that gives rise to a virtual antenna pattern of a particular shape, by using a number of individual antenna elements that form part of the overall antenna. The antenna pattern is typically shaped to give rise to a “beam” directed towards a particular user. This maximizes the antenna gain in this particular direction. In addition, the pattern of the beam can be shaped to guarantee minimum gain in the direction of unintended users, which has the additional benefit of reducing interference in the network. A beam-forming unit calculates the phase and amplitude offsets by which the antenna elements should be driven, to give rise to a particular shape of antenna pattern. This is achieved by adjustment of the azimuth, and possibly the elevation angles, to the intended and unintended uses and may be augmented by feedback from the mobile station. In general, the greater the number of antenna elements the more accurate and precise the beam pattern formed.
A multicast transmission can be broadcast to a user group or groups in a particular cell that consists of a limited number of users that can employ feedback from the terminal to adapt the multicast transmission. The feedback can be in the form of a request to re-transmit transport blocks, such as for example an automatic repeat request (ARQ) non-acknowledgement message (NACK) (also referred to as an ARQ-NACK). This may be used in, for example, a GSM (Global system for mobile communication), UMTS (Universal Mobile Telecom System) or LTE (long term evolution) multicast-broadcast multi-service (MBMS) environment. In a multicast transmission environment with ARQ, including hybrid ARQ (HARQ), any user which has not received the transport block correctly can transmit an NACK message so that the base station will re-transmit the missing transport blocks. This may take place with different modulation and coding schemes. The re-transmission may then be combined in the receiver with previous transmissions in order to enable successful decoding of the transport block. This process may be repeated until a predefined criterion is achieved. For example, until all terminals in the group have correctly received the transport blocks; or until a predetermined maximum number of re-transmissions have been reached.
Beam-forming is usually applied to transmissions intended for a single user. In the case of a multicast for multiple users, all should be able to receive the same transmission simultaneously. If the multicast is broadcast to users which are randomly distributed within a particular cell, it is generally not possible to produce an antenna pattern shaped in such a way that all users receive a transmission of sufficiently high gain. This is due to the fact that the antenna pattern includes notches in certain directions between some of the users. In general the number of users that can be addressed with a unique antenna pattern tends to be equal to the number of elements in the antenna array. As a result, for groups of more than a few users, beam-forming is not applied to multicast transmissions.
US 2003/00 647-5481 (Wilson et al) discloses a system and related method for clustering multi-point communication targets. This patent application discloses building a set of clusters including one or more targets, and sharing a wireless communication channel with the clusters based at least in part on performance characteristics of the targets, such as the spatial signature. The target may be allocated or reallocated to one or more clusters as appropriate and performance characteristics of the targets are monitored in order to reinitiate the relative weight of a target within a cluster. This patent application does not teach how to address the issue of a target not receiving a transmission and the subsequent solution to that problem.